Field of the Invention
The invention relates to a steering system having a displacement unit and a control unit including an inner control piston and an outer control sleeve, wherein both the control piston and the control sleeve include axially and radially extending channels which communicate with each other and with the displacement unit. The channels form a main conduit extending from a supply connection through the displacement unit and a steering cylinder to a return connection, in which main conduit there are disposed an adjustable input throttle between the supply connection and the displacement unit, two adjustable throttles between the displacement unit and the steering cylinder, and an adjustable return throttle between the steering cylinder and the return connection. A first side conduit extends from the main conduit downstream of the adjustable input throttle to the return connection, in which first side conduit there are disposed a first and a second adjustable load signal throttle, and a second side conduit extending from the load signal connection to the first side conduit at the height between the first and the second adjustable load signal throttle. In the neutral position the first adjustable load signal throttle has a positive overlap as compared to the other adjustable throttles. Steering systems of this type are used in mobile installations which include a steering loop and an operating loop wherein both loops have a common supply pump and the steering loop has priority.
Installations of this type consist, on one hand, mainly of a common supply pump and a common flow division control valve as well as of the steering system and steering cylinder, and, on the other hand, of an operating loop. The flow division control valve is spring biased and receives its control signal in form of a so-called load signal from the hydraulic steering system. The flow division control valve regulates its flow cross sections based on the load signal such that the steering loop is assigned a higher priority and supplied with hydraulic oil as needed, whereas the operating loop receives the excess hydraulic oil. It is known that the load signal can be processed statically or dynamically. Due to the fast reaction speed of the dynamic load signal, the latter processing method is becoming more prevalent in practical applications.
A steering system of this type is described, for example, in the German Patent Application P 44 10 693.9 by the applicant. It consists of a displacement unit and a control unit with an inner control piston and an outer control sleeve provided with channels communicating with each other and with the displacement unit. The channels form a main conduit extending from the input connection to the displacement unit, with an adjustable input throttle interposed therebetween, another main conduit containing adjustable throttles and extending from displacement unit to the steering cylinder, and another main conduit including an adjustable return throttle extending from the steering cylinder to the return connection.
The channels furthermore form a first side conduit which extends from the main conduit downstream of the adjustable input throttle to the return connection and includes a first and second adjustable load signal throttle and a constant throttle.
The channels, in addition, form a second side conduit which extends from the load signal connection to the first side conduit and discharges between the first and the second adjustable load signal throttle.
The adjustable input throttle, the adjustable return throttle and the second adjustable load signal throttle close in the same direction. The first adjustable load signal throttle closes in the opposite direction as the aforementioned adjustable throttles.
This steering system is rather sophisticated and has significant advantages. It has, however, the same disadvantages as any other steering system controlled by load signals. When the steering is not operated, the entire load signal flow has to be regarded as wasted, and even when the steering excursions are small, there is still a certain portion of the flow wasted, all of which has a negative effect on the energy efficiency of the hydraulic installation.
All steering systems with a closed-center design have a further disadvantage. When the steering cylinder of a closed-center steering system is pulled by an external force in the steered direction and the control unit of the steering system is steered from the neutral position into the corresponding operating position, the steering cylinder moves in advance of the apportioned flow of the hydraulic oil, causing the steering response during this phase to be uncontrolled.
A steering system of similar design is known from EP 0561 401 A1. This steering system, however, is provided with an open center and includes a connection for a pressure signal from a pressure switch for actuating the electric motor of the hydraulic pump.
The steering system is consequently not suitable for the intended application.